A Size 3 avalanche, which could bury or destroy a car or house and break trees, came down in Hero´s Bowl in Kananaskis Country this week. (Photo courtesy Kananaskis Country Public Safety).Ted Rhodes
/ Calgary Herald

A naturally triggered Size 3 avalanche came down on Tuesday morning off the east face of Snow Peak and crossed the meadow on the way to the final slopes to gain the top of Burstall Plass. The debris ran across the flats and into the trees. (Photo courtesy Kananaskis Country Public Safety)Ted Rhodes
/ Calgary Herald

Officials with Kananaskis Country, Parks Canada and the Canadian Avalanche Centre took the unusual step this week of issuing a special avalanche warning for the mountains in British Columbia and Alberta.

“It’s a fairly complex, tricky scenario,” said Karl Klassen, avalanche warning service manager for the Canadian Avalanche Centre, based in Revelstoke, B.C.

The warning, based on what several forecasters have called “spooky” conditions, recommended recreational backcountry users avoid avalanche terrain and suggested experienced backcountry recreationists travel on simple terrain with no large steep slopes. All avalanche terrain should be avoided in sunny conditions.

Klassen said the situation comes from a confluence of events, starting with a lack of snow early in the season.

“When we have a shallow snowpack like that early in the season and it lies on the ground exposed to the atmosphere for an extended period of time, it often becomes weak,” he said.

Then the cold hit, forming a sugary snow that doesn’t bond well. When the storms arrived last weekend, dumping between 40 and 70 centimetres of snow in the Rockies, it landed on top of the weak, sugary layers.

“Obviously that’s an unstable situation,” said Klassen.

Many natural avalanches have occurred and others have been triggered by officials this week in the mountain parks.

Steve Holeczi, a visitor safety specialist with Parks Canada, said avalanches were remotely triggered along the highways throughout Banff, Yoho and Kootenay.

“We’ve been getting really big results,” he said. One avalanche across Highway 93S on Thursday brought down trees and other debris with little effort.

Holeczi said they’ve also seen lots of natural avalanches in the backcountry as they flew overhead.

“There are large avalanches everywhere,” he said.

In Kananaskis Country, crews have witnessed several large, natural avalanches.

“They’re coming right from the high ridges, the alpine terrain up high, running right to the valley floor,” said Burke Duncan, a public safety specialist with Kananaskis Country. “In some cases, it’s running across the valley floor and up the slopes on the other side.

“Those are large events and they are full depth events so the whole winter snowpack is failing. Basically it’s failing right to ground.”

The natural cycle will likely drop off, but officials said the rise in temperatures throughout Western Canada this weekend is leading to a new set of concerns.

“We’re looking at sunny conditions and clear skies,” said the avalanche centre’s Klassen. “When you get these very warm temperatures and solar radiations treating the snowpack, the snow warms up and warm snow is not as stable as cold snow.

“So now we have this destabilized or weakening snow on the surface.”

Klassen believes the unstable snow on the surface will start sliding off in the warm weather.

“It’s going to shake up or impact or overload the layers that still exist deeper in the snowpack,” he said, which will cause the top 30 to 40 centimetres to slide. “In certain places, those surface avalanches are going to trigger deeper avalanches and we’ll get what we call step-down avalanches.”

A step-down avalanche starts on the surface, overloads the layer deeper in the snowpack and breaks much deeper and much broader.

“When you have that, it’s a little less predictable,” he said.

Klassen said the final piece of the puzzle was this weekend’s forecast for blue skies and high temperatures.

“People tend not to perceive hazard or risk as acutely during nice, sunny warm days as they do on stormy, cold days,” he said.

“Every now and then, you are going to get this massive avalanche that is going right to the ground.

“It’s going to go hundreds of meters wide and it’s going to be very, very destructive.”

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